@ailmam as a step parent myself I’m going to say this:
You need to think about what it is that’s making you uncomfortable when your DP’s DD is around.
Is it the fact that you can’t relax, do you feel as though you are watching what you are saying/doing, careful not to hog her dads attentions, not being as affectionate with DP as not wanting to make her uncomfortable? Does she perhaps want a lot of attention from you?
OR
Is it that you don’t like her? Or that there is something about her you find annoying/trying behaviour?
OR
Is is to do with how your DP might change when she is around? Does he baby her? Does he make you feel excluded?
I’m not saying you have to answer these here, just that you should think deeply about what it is that is bothering you. Knowing how you feel and being able to pin point where the negative feeling is coming from will give you a clue on whether it’s a temporary feeling/fixable or if you should cut your losses and move on.
Keep in mind also that it’s been lockdown and the kid hasn’t seen or socialised with her friends and peers like normal. Having other people to spend time with, and a woman who isn’t her mum might be quiet refreshing for her at the moment. Come ages 13+ lots of teens want to spend most of their time with friends and often become less dependent on their parents time as they start to go off and do activities with their mates instead.
Forging step relationships can be hard especially with older children. It’s a very good start that his DD likes you and wants to spend time around you. It can take time to find your groove with it, but it definitely isn’t for everyone, there is a lot of thankless sacrifices that step parents make, but it can be really rewarding too.